Solenoids are common, and have many practical applications where an axial magnetic field is desired. Such practical applications include such devices as electron beam tubes. Typically, solenoids are made of a long wire wound in a close packed helix forming a cylindrical tube. When current is passed through the wire, a magnetic field is created. If the length of the solenoid is long compared to its diameter, an axial substantially uniform magnetic field is created within the bore of the solenoid. The magnitude of this magnetic field is controlled by the current within the wire forming the solenoid. The larger the current, the greater the magnetic field created. Therefore, for applications that require predetermined magnetic fields, the magnetic field necessary is generated by controlling the current within the wire forming the solenoid.
With the proliferation of superconductivity and its resulting practical applications, difficulties have arisen. When a persistent current flows in a superconducting solenoid, thereby forming an axial magnetic field within, the persistent current is not easily controlled once it has been established. For this reason, difficulties have been encountered in adjusting the magnetic field within a superconducting solenoid once the persistent current has been established. Therefore, there is a need for controlling the magnetic field generated by a superconducting solenoid once a persistent current is established therein.